It's an activity tracker you can just clip to your waist band or bra strap day n night and it tracks what you do. It connects wirelessly to your pc and you can then see your sleep pattern, how often you wake-up or roll around. You can also specify what activity you were doing swimming, running etc as different activities burn diff e rent amounts of calories.
I have one and when I was on a diet I loved it, wore it everyday and it motivated me. Alongside 'my fitness pal' app where I tracked my food intake I was able to lose 5 stone in 6 months, build my stamina etc. Then I got pregnant, now I have a toddler and I'm fat and unfit again